1. Field of the Invention
This invention relates generally to physical vapor deposition (PVD) coating systems and processes, and more particularly to sputtering machines utilizing one or more pairs of dual cylindrical rotary magnetrons operated in a manner which gives high deposition rates with excellent process control and long term operating stability. The design of the sputtering apparatus and process makes it especially useful for depositing electrically insulating (dielectric) and semiconducting coatings on a variety of substrates. Products benefiting from this technology include but are not limited to computer recording heads, flat panel displays, integrated circuits, computer memory disks, and a variety of coated glass products.
2. Brief Description of the Prior Art
The methods commonly used for depositing thin film coatings in vacuum generally can be classified as physical vapor deposition (PVD) or chemical vapor deposition (CVD) processes. PVD methods can be divided into evaporation (thermal energy source) and sputtering (plasma energy source). Both evaporation and sputtering have enjoyed successful application in various product lines. Evaporation has been used mostly for coating relatively small substrates. Some examples of high tech products include eyeglasses, filters, optical windows of many types, lenses, and laser optics. These products rely heavily on dielectric coatings for their functionality. Large evaporative coating machines are used for less sophisticated metallic coatings on rolls of substrate. Typical products include food packaging, decorative coatings, and the surface coating (corrosion treatment) of rolls of steel.
The sputtering phenomenon was recognized before evaporation, but evaporation technology was developed first because sputtering deposition rates were initially so low that it was not considered useful. A familiar example of low sputtering rate is the slow erosion of the tungsten filaments of ordinary incandescent light bulbs. They are very slowly sputtered by the argon gas used to fill the bulb to protect the hot filament from oxidation. The material removal rate is so slow that only a slight discoloration is noticed on the inside of the glass envelope after many hours of use. The early development of sputtering technology used a simple cathode and anode (diode) placed in a vacuum system with the cathode formed from the metal to be sputtered. Direct current (DC) in the range of 1,000 to 20,000 volts was the most commonly used power source. The process was able to produce an opaque metal coating on glass (a mirror) in about an hour to several hours depending upon the metal used. A description of the early sputtering process is given by J. Strong in xe2x80x9cProcedures in Experimental Physicsxe2x80x9d copyrighted in 1938.
Sputtering became a more commercially viable process with the invention of the planar magnetron, U.S. Pat. No. 4,166,018 issued to Chapin and entitled xe2x80x9cSputtering Process and Apparatusxe2x80x9d. This patent described a sputtering apparatus in which a magnetic field is formed adjacent to a planar sputtering surface with the field comprising arching lines of magnetic flux over a closed loop erosion region on the sputtering surface. A key enabling feature of the device was that the magnetic field lines penetrated the target material. The erosion region took the form of an annular erosion zone over the target material. The annular erosion zone is referred to, among those skilled in the art, as the xe2x80x9cracetrackxe2x80x9d because of its elongated shape on a rectangular planar magnetron. On round planar magnetrons, the xe2x80x9cracetrackxe2x80x9d usually has a circular shape, but it can have other shapes as dictated by the design of the magnet array so long as the racetrack is a closed loop. The magnetrons were almost always operated with DC power, and the phrase xe2x80x9cDC Magnetron Sputteringxe2x80x9d became the way the process was usually referred to. The erosion process caused heating in the annular erosion zone, which had to be removed by cooling the target material either directly or indirectly with water.
Industrial use of DC magnetron sputtering for the deposition of metallic films increased during the late 1970""s and early 1980""s. Large sputtering machines were built to coat solar heat reducing reflective films on windows for large commercial buildings and for other applications. For large substrates, it was superior to evaporation for controllability and uniformity. And, although the deposition rates were not as high as for evaporation, sputtering had become an economically useful process. Initially, in the erosion zone, early magnet arrays created a relatively sharp cross-section in the target material. A large metal target plate would quickly xe2x80x9cburn through,xe2x80x9d resulting in very poor utilization of the material. Increased target life was achieved (i) by improving the design of the magnet array to widen the erosion zone on the surface of the target, (ii) by increasing the magnetic field strength to operate with thicker targets, and (iii) by mechanisms which moved the arrays to widen the erosion zone during operation. U.S. Pat. No. 5,262,028 issued to Manley and entitled xe2x80x9cPlanar Magnetron Sputtering Magnet Assemblyxe2x80x9d is an example of an improved design for the magnet array to widen the erosion zone. U.S. Pat. No. 5,417,833 issued to Harra et al and entitled xe2x80x9cSputtering Apparatus Having a Rotating Magnet Array and Fixed Electromagnetsxe2x80x9d is an example a circular planar magnetron with a rotating magnet array to improve target utilization.
Another approach to improving the utilization of the target material was taught in U.S. Pat. No. 4,356,073 issued to McKelvey. The target material was formed into a straight (constant diameter) tube, which was rotated around its cylindrical axis with internal racetrack magnets that were held fixed with respect to the substrate. In this embodiment the target material (the tube) becomes thinner as the process proceeded and no xe2x80x9cburn throughxe2x80x9d in the erosion zone occurs. Its initial use was for the large scale coating of metal films on architectural glass by DC magnetron sputtering. The magnetron coated only one side of the glass substrate in the horizontal position. Later U.S. Pat. No. 4,445,997 again issued to McKelvey disclosed another rotatable sputtering magnetron in which the target tube was contoured longitudinally to match the geometry of a substrate with the shape of an automobile windshield. This magnetron was also designed for use in the horizontal position. In U.S. Pat. No. 4,466,877 McKelvey describes a pair of rotatable cylindrical magnetrons mounted horizontally and spaced in a parallel relationship to each other such that the sputtered flux directed inwardly and downwardly from each is focused on the same region of the substrate. Further details of this dual embodiment are given in an article by Shatterproof Glass Corporation entitled xe2x80x9cRotatable Magnetron Sputtering Sourcexe2x80x9d, Solid State Technology, April 1986. Two important points from the article are first, the magnetrons are rotated at speeds from 1 to 12 RPM, and second, the horizontal mounting for sputtering downward on glass causes the target cooling water to be fed through two rotating seals in vacuum, causing reliability problems and limiting rotation speeds.
In addition to the sputtering of metal films, DC reactive magnetron sputtering has been developed for the deposition of insulating (dielectric) and semiconducting coatings; in particular, for the deposition of the oxides and nitrides of metals. In reactive sputtering, the inert working gas is usually argon and the added reactive gas is often oxygen and/or nitrogen. The coating of dielectric materials can be accomplished by RF sputtering of the dielectric material itself used as the target. However, in both RF diode and RF magnetron modes the deposition rates are very low. Despite the low rates, this form of sputtering is still used in the production of thin film recording heads and integrated circuits. DC reactive magnetron sputtering of insulating films has the potential advantages of higher deposition rates and lower costs, but good process control and long term stability have been difficult problems to solve for the deposition of the highly insulating metal oxides and nitrides.
There are three characteristics of reactive DC magnetron sputtering that cause issues with control and process stability. One characteristic is generally referred to as xe2x80x9ctarget poisoningxe2x80x9d by the reactive gas. Poisoning is the term used to describe the phenomenon where, for example, oxygen reacts with the metal target surface to form an oxide. Sputtering rates for oxides are usually an order of magnitude or more less than that for the pure metal. The natural tendency for the magnetron is to be most stable in the xe2x80x9cmetal modexe2x80x9d or the xe2x80x9coxide modexe2x80x9d (poisoned), while the desired operating point is often between the two where the process is highly unstable. One reason for the instability is that the partial pressure of the reactive gas (e.g. oxygen) that is required at the substrate to form the insulating film is also adequate to substantially poison the target. Conventionally, some means of partial pressure separation of the reactive gas between the magnetron and the substrate has been attempted to achieve process stability. Examples include U.S. Pat. No. 4,851,095 issued to Scobey et al; U.S. Pat. No. 4,420,385 issued to Hartsough; Scherer et al, xe2x80x9cReactive High Rate DC Sputtering of Oxidesxe2x80x9d, Thin Solid Films, Vol. 119 (1984) 203-209; Schiller et al, xe2x80x9cReactive DC Sputtering with the Magnetron-Platatron for Titanium Pentoxide and Titanium Oxide Filmsxe2x80x9d, Thin Solid Films, Vol. 63 (1979) 369-373; and Schiller et al, xe2x80x9cAdvances in High Rate Sputtering with Magnetron-Plasmatron Processing and Instrumentationxe2x80x9d, Thin Solid Films, Vol. 64 (1979) 455-467. These techniques cover the range from placing baffles partially between the magnetron and substrate and injecting the reactive gas only near the substrate, to completely separating the deposition region from the reaction region by moving the substrate. The method of Hartsough was improved and expanded by Scobey. A metallic film was deposited on substrates in one area of the machine and the substrates were then moved to another area of the machine for reaction (e.g. oxidation) to form a transparent optical film. Although they obtained relatively high net deposition rates, they were still limited by the reaction rate of the metallic film with oxygen that could be achieved in the separated reaction zone. Additionally, this separation of zones clearly limits the number of magnetrons and reaction zones which can be placed in a given vacuum chamber to half of the number of magnetrons which could otherwise be used, thus further limiting the net deposition rate of the system. In many of the examples the films required a post operation bake at elevated temperatures to fully complete the reaction.
An elaborate gas separation scheme is described in U.S. Pat. No. 5,338,422 issued to Belkind et al. It teaches a triple magnetron array consisting of a rectangular planar and two rotatable cylindrical magnetrons with unbalanced magnet arrays. The planar magnetron is partially shielded by a baffle from the rotatables, and argon (inert working gas) is injected near the planar which is operated in metal mode, while oxygen is injected near the rotatables. In another example of a gas separation apparatus U.S. Pat. No. 5,384,021 issued to Thwaites describes a machine in which a single rotatable cylindrical magnetron is augmented with supplementary magnets and reactive and ionizable gas remote form the sputtering zone.
A second control and process stability problem involves changes in the electrical circuit of the magnetron. With a few exceptions most metal oxides and nitrides are good electrical insulators. During operation, insulating material builds up on the shields, walls, and other structures in the vacuum system, causing the electrical resistance through the plasma to the anode and any other return path back to the power supply to become higher and higher. This shift in the electrical resistance in the power circuit causes a process drift, which is difficult to control while keeping the film properties constant with time. U.S. Pat. No. 5,169,509 (""509) issued to Latz et al describes a dual rectangular planar magnetron apparatus operated by ungrounded AC power in the frequency range of 1 kilohertz to 100 kilohertz. No reference was made in ""509 of the ability of AC operation to help minimize the problem with changes in circuit resistance, but it is obvious in retrospect that they were on the right track. In fact, a paper by Scherer et al (including Latz) entitled xe2x80x9cReactive Alternating Current Magnetron Sputtering of Dielectric Layersxe2x80x9d that was publish soon after ""509 was filed, clearly pointed out the well defined anode characteristics of the AC operation. More recently, U.S. Pat. No. 5,814,195 (""195) issued to Lehan et al discusses the advantage of using AC power on dual rotatable magnetrons for anode stabilization in a very similar manner to that described by Scherer et al.
A third characteristic of reactive DC sputtering, which causes process difficulties, is the arcing phenomenon. While insulating material is building up on various structures in the vacuum system, it is also building up on non-sputtered regions of the sputtering target. This is more severe for pairs of magnetrons that are operated facing each other in order to coat both sides of a substrate simultaneously. During operation the insulating regions are constantly being bombarded by ionized inert sputtering gas (e.g. argon) which removes electrons and leaves a net positive charge on the insulating surface. Eventually, the voltage difference between the negatively charged target plate and the positively charged insulating surface reaches breakdown potential for the insulator and arcing occurs. This arcing has at least two negative consequences. First, it can disrupt the process control system, and second, it can damage the coating in a number of ways. Physical damage can occur by the arc striking the coating, the ratio of metal atoms to reactive gas atoms (stoichiometry) in the coating can be changed in the vicinity of the arc, and the arc can create particulate debris, which can get into the coating. Rotatable cylindrical magnetrons help to minimize the arcing problem because the build up of the dielectric on the active part of the target is largely prevented by the rotation (if it is rapid enough). However, there is buildup of dielectric along the outer edges of the sputtering region where arcing can still occur. Since the rotatable magnetrons are primarily used in horizontal sputter down orientation, particulate debris can be a major problem for many coating applications. AC operation of the magnetrons, as described in ""509 and ""195, helps to reduce arcing because the positive charge buildup on the insulating surface tends to be neutralized on each half cycle. The paper by Scherer et al explains the details of the arcing phenomenon in sputtering, and the improvement that AC operation provides. But, the arcing is not totally eliminated largely because the buildup of insulating material on the non-sputtered regions of the planar target surface becomes excessive after long periods of operation. The improvement of ""195 simply substitutes rotatable magnetrons for the planar magnetrons of ""509 in order to reduce the buildup of insulating material on the target surface because of the continuous rotation of the target surface past the sputtering zone. While the buildup of insulating material on rotatables is most severe at the ends of the sputtering zone, it is still significant on the sputtered surface. Indeed, the method of ""195 relies upon connecting the power supply to the magnetron through a transformer and an impedance-limiting capacitor to reduce arcing. The current art still fails to provide a completely satisfactory solution to arcing for long term operation.
The deposition of the protective overcoat for computer memory disks is a good example of how the arcing problem affects a product. Carbon is sputtered in argon plus hydrogen or a hydrogen containing gas to form a hard. insulating xe2x80x9cdiamond like carbonxe2x80x9d (DLC) protective overcoat layer. Both sides of the disk are coated simultaneously by pairs of facing magnetrons, so the buildup of insulating material on the non-sputtering regions of the target surfaces is relatively rapid. Particulate debris caused by arcing is a major source of reduced yields industry wide. AC operation offers some improvement but has done little to eliminate the problem.
Another problem with the current art is that the geometrical arrangement of the magnetrons with respect to the substrate in coating machines can cause variable stoichiometry in the deposited insulating film. Most often the substrate is caused to pass by and below the magnetron, in a direction perpendicular to its length, by some mechanical transport means. Since the partial pressure of the reactive gas near the substrate is constant, the instantaneous deposition rate over the deposition region also needs to be constant if the resulting film is to have uniform stoichiometry. This is not normally the case, and to correct it in the current art requires shielding the deposition region to a narrow slit across which the deposition rate is approximately constant. This lowers the efficiency of the use of the available deposition flux by more than fifty percent. Variability of coating stoichiometry has not been a problem for optical films (so long as they are clear), but it can be a significant issue for products that require subsequent processing, or have requirements related to levels of stress. Post deposition chemical etching of the coating is an example in which variable film stoichiometry would not be acceptable. Also, semiconducting films which must be optically transparent and maintain a modest but uniform electrical conductivity, need to have uniform stoichiometry.
Process control for reactively depositing insulating films at high rate traditionally has been very difficult. Baffles, which provided some separation of the partial pressure of the reactive gas between the target and the substrate, allowed operation for a short time at the edge of the xe2x80x9cpoisonedxe2x80x9d mode, with only slightly improved deposition rates when compared to no baffling. However, invariably the process drifts due to vacuum system cleanup, contamination drag in from the substrates, target erosion, substrate motion, and other factors. An improvement in process control for depositing insulating films was described in ""509, which used a feedback circuit to vary the flow of reactive gas to hold the plasma potential at a constant desired value. This gave improved control and increased deposition rates, but the feedback loop is unstable against arcs, and for some films, aluminum oxide for example, there are two values of voltage corresponding to two different film compositions for the same reactive gas flow. The system is unstable for these types of films, and can transition quickly from one voltage to the other.
A prior art sputtering apparatus for the deposition of dielectric materials using an AC power supply is described by Lehan et al in the previously cited U.S. Pat. No. 5,814,195. FIG. 1a is the schematic diagram of the AC powered sputtering system 1 of Lehan et al. A chamber 2 is fitted with a pair of horizontally mounted rotatable cylindrical magnetrons 3a supporting tubular targets 3b positioned over substrate 4. The two poles of an AC power source 5 are connected to a transformer 6, whose output is connected to magnetrons 3a, 3b through fixed and variable impedance-limiting capacitors 7. Magnet assemblies 8 create sputtering erosion zones on the surfaces of the tubular targets. This apparatus is practically identical to that taught by Latz in ""509 with the planar magnetrons of ""509 simply being replaced by rotatable cylindrical magnetrons. Lehan et al in ""195 claim that the rotation of the target past the erosion zone cleans the dielectric buildup material off the target in portions away from the erosion zone to help reduce arcing. While this is an obvious aid in reducing arcing, a better solution which eliminates arcing would not require the transformer 6 and capacitors 7.
FIG. 1b is the enlarged schematic diagram of magnet assemblies 8 inside tubular targets 3b of FIG. 1a. Magnets 9a, 9b, and 9c in one tubular target and magnets 10a, 10b, and 10c in the other tubular target produce parallel containment areas (for electrons) 11a and 11b that form erosion zones defined by magnetic fields 12a and 12b. Nothing is specified about the polarity of the magnets, except by implication from the schematic shape of the magnetic fields, magnet 9b is opposite to 9a and 9c and 10b is opposite to 10a and 10c. In the figure all of the magnets point radially away from the geometrical centers of tubular targets 3a. Distance 13 between the magnetrons is stated to be about 1-3 inches, while distances 14 and 15 from the (grounded) chamber walls should be about four or more inches. This geometrical arrangement causes the sputtered flux to be non-uniform across the deposition region at the substrate.
Prior art sputtering equipment and processes fail to provide an economical and workable solution for obtaining controllable high rate deposition of insulating and semiconducting films in a demanding (especially high tech) manufacturing environment. Each of the methods previously described suffers from one or more of the problems commonly associated with reactive sputtering. What is needed is an improved sputtering apparatus and control process for high rate deposition of insulating and semiconducting films that does not require complex geometrical baffling for reactive gas partial pressure separation, physically separate coating and reaction zones, arc suppression circuit additions, or post deposition heat treating. In applications for high tech products, the particulate debris from all sources including arcing and coating flaking from baffles and shields must be reduced to an absolute minimum; and, the film stoichiometry must be kept uniform throughout the thickness of the film. Additionally, the process must minimize heating of the substrate during deposition.
All of the patents and references cited above and U.S. Pat. No. 443,110 dated Jul. 27, 1998 and U.S. Pat. No. 444,066 dated Sept. 14, 1998 are hereby incorporated by reference for purposes of additional disclosure.
Accordingly, it is an object of the present invention to provide an apparatus and process for coating substrates with dielectric or semiconducting films at high deposition rates.
A second object of the present invention is to provide a deposition process that is stable and can be accurately controlled over extended periods of operation of the apparatus.
Another object of the present invention is to provide an apparatus and process, which produces dielectric and semiconducting films with controlled and uniform stoichiometry.
Yet another object of the present invention is to provide an apparatus and process for depositing insulating films with greatly reduced levels of particulate debris from arcing and flaking.
In accordance with one aspect of the present invention the deposition apparatus comprises at least one set of improved dual rotatable cylindrical magnetrons mounted vertically in a vacuum chamber. Associated with the set of magnetrons are baffles, shielding, a vacuum pump (or pumps), and appropriate valving which together constitute a semi-isolated sputtering enclosure or module. The preferred vertical mounting of the magnetrons (in the direction of the gravitational force) allows the target tubes to be relatively long compared to their diameter, so they can be mounted in close proximity to each other without intermittent contact and cracking caused by mechanical deformation. The magnet assemblies are intentionally, (i.e. contrary to the conventional art) designed to provide very narrow erosion zones along the parallel sides of the racetrack (the parallel sides of each racetrack approaching a pair of line sources) for each magnetron. These parallel erosion zones have a highly concentrated plasma density for rapid sputtering of the target and any reactive material. Also, they enable sputtering at low pressures, which yields films formed at higher average energy per deposited atom. The very narrow, high plasma erosion zones allow a lower threshold rotational speed for acceptable magnetron operation because they are effective in keeping the magnetron operating in metal mode even in the presence of a relatively high partial pressure of reactive gas. The distance between the narrow, high plasma parallel erosion zones, the placement of the vertically mounted substrate with respect to the magnetrons, and the pointing angles of the racetracks toward the substrate and each other, are optimized to form a relatively wide and efficient (above 75%) constant flux deposition region at the substrate. This allows high deposition rates at constant reactive gas partial pressures with substantially uniform film stoichiometry. The rotatable cylindrical design, if properly implemented, can have high flow rates of cooling water to allow sputtering at higher energy densities and higher deposition rates than is possible on typical planar magnetrons. Vertical mounting of the magnetrons minimizes substrate particulate contamination due to flaking from shields and adjacent structures. A very significant improvement in design is the elimination of all rotating water seals in vacuum, which provides reliable operation at rotation rates higher than 1 to 12 RPM.
According to another aspect of the present invention, several beneficial properties are obtained from the operation of the dual rotatable magnetrons with AC power in the frequency range of approximately ten to hundreds of kilohertz. In this mode the poles of the AC power supply are connected directly across the electrodes of the dual rotatable magnetrons which are electrically floating (not grounded). The combination of the AC operation with the high plasma density erosion zones previously described insure that when one magnetron is sputtering on the negative cycle of the AC power, the other magnetron provides a low and constant electrical resistance return path to the power supply. Both the high power density of the plasma and the proximity of the racetrack erosion zones, due to the racetracks being angled toward each other, provide greatly improved coupling between the magnetrons in comparison to ""751 and ""195. As a result of this arrangement no other part of the vacuum system mechanical structure is required to be electrically conductive, and therefore such structures could be made entirely from insulating materials if desired. It may be desirable for the structures near the magnetrons to have electrically insulating surfaces (i.e. a coating) to insure that the process remains electrically constant from startup. The frequency of the AC power may be selected (over a wide range) to minimize material specific arcing, and to provide more efficient formation of monatomic species from diatomic reactive gas molecules by taking advantage of frequency resonances in their dissociation characteristics.
According to still another aspect of the present invention, process control is greatly improved by operating the AC power circuit at a constant pre-selected voltage while the pumping speed of the system, the flow of sputtering gas, and the flow of reactive gas are also held constant. It is found that these parameters fix the stoichiometry of the depositing film, and the process will remain stable over long periods of operation. The process has minimal sensitivity to the way the reactive gas is added to the system in the sense that elaborate schemes of baffles to provide physical separation of the partial pressure of the reactive gas between target and substrate are not needed. If several sets of dual magnetrons are to be operated in the same vacuum chamber, it is advantageous to array them in semi-isolated enclosures with dedicated pumps and baffles between the different sets of magnetrons (i.e. to form modules as previously described) to reduce any cross talk between control loops.
An advantage of the present invention is that the new apparatus and process can produce electrically insulating and semiconducting coatings at very high deposition rates for extended periods of time.
Another advantage of the present invention is that the electrically insulating and semiconducting coatings have a high degree of stoichiometric uniformity as compared to the coatings produced using the prior art.
Yet another advantage of the present invention is that it provides dielectric coatings, produced by high energy, low-pressure deposition process, which have superior density, hardness, and adhesion.
A further advantage of the present invention is that the use of baffles to maintain partial pressure separation of the reactive gas between the target and the substrate, and the injection of the reactive gas only near the substrate can be eliminated.
Still another advantage of the present invention is that it can process multiple wafer substrates simultaneously at high throughput, thus reducing manufacturing and clean room floor-space requirements by approximately an order of magnitude for the same total output.